FIG. 6 is a block diagram showing a main circuit of a three-phase neutral point clamped power conversion apparatus connected with a load in the form of a motor. The power conversion apparatus controls switching devices S1˜S12 (IGBTs in FIG. 6) ON/OFF by inputting gate commands to the switching devices S1˜S12, and thereby outputs an ac voltage at output terminals U, V and W.
This power conversion apparatus is arranged to divide a dc voltage P-N by the use of smoothing capacitors Cdc1 and Cdc2, and to produce an output with a PWM (Pulse Width Modulation) of dc potentials P and N and a neutral point potential NP at three levels.
Patent documents 1 and 2 both employ a neutral point potential control (Neutral Point Control) for suppressing unevenness in the dc voltages Vdc1 and Vdc2 after ACR (current control: Automatic Current Regulator). The neutral point potential control is a control to determine a deviation between the dc voltage Vdc1 of smoothing capacitor Cdc1 on the positive side (P side) of the dc voltage P-N and the dc voltage Vdc2 of the smoothing capacitor Cdc2 on the negative side (N side), and to perform a control operation to reduce the deviation toward zero.
A control circuit of the power conversion apparatus is explained hereinbelow in the example in which the three-phase ac output of this power conversion apparatus is connected to the load in the form of a motor M, and a current control is performed. FIG. 7 is a block diagram showing the control circuit of the three-phase neutral point clamped power conversion apparatus.
A phase detector enc is attached to the motor M and arranged to detect a phase and to provide a detected phase theta_det. A three-phase to two-phase converter 1 converts output currents (motor currents) Iu, Iv, Iw by three-phase to two-phase conversion on the basis of the detected phase theta_det, and thereby provides detected d-axis current Id_det and q-axis current Iq_det.
Then, a current control section 2 performs a PI control to cause the detected d-axis current Id_det and detected q-axis current Iq_det to follow respective target values set equal to a command d-axis current Id_cmd and a command q-axis current Iq_cmd, respectively. On the output side of the PI control, there are provided limiters LMT1 and LMT2 for restraining integral outputs at the time of output saturation and thereby stabilizing the system. The output of current control section 2 is a two-phase command voltage of Vd_cmd and Vq_cmd.
The two-phase command voltage Vd_cmd, Vq_cmd is converted, by the two-phase to three-phase conversion with a two-phase to three-phase converter 3, into a three-phase command voltage V_cmd. Next, a neutral point potential control section 4 adds a neutral point potential compensation quantity V_cmp to the three-phase command voltage V_cmd, and thereby delivers the command voltage obtained by the addition as a corrected command voltage V_cmd′.
Furthermore, a limiter LMT3 imposes voltage limitation on the corrected command voltage Vcmd′ and supplies a limiter processed command voltage Vcmd″ obtained by the voltage limitation, to a PWM process section PWM. The limiter LMT3 is provided to prevent later-mentioned gate commands GI_H and GI_L from becoming abnormal pulse (minimal pulse, for example) and to prevent the output voltage and output current of the power conversion apparatus from being distorted.
The PMW process section PWM produces gate commands GI_H, GI_L for each of switching devices S1˜S12 by using the limiter processed command voltage Vcmd″. Generally, the gate commands GI_H, GI_L are produced by comparison between a triangular wave carrier signal and the limiter processed command voltage V_cmd″, though not shown in FIG. 7.
The three-phase to two-phase conversion in the three-phase to two-phase converter 1, and the two-phase to three-phase conversion in the two-phase to three-phase converter 3 are represented by following expressions (1) and (2).
                              [                      Math            .                                                  ⁢            1                    ]                ⁢                                                                                                [                      C            dq                    ]                =                                                            2                3                                      ⁡                          [                                                                                          cos                      ⁢                                                                                          ⁢                      θ                                                                                                  sin                      ⁢                                                                                          ⁢                      θ                                                                                                                                                          -                        sin                                            ⁢                                                                                          ⁢                      θ                                                                                                  cos                      ⁢                                                                                          ⁢                      θ                                                                                  ]                                ·                      [                                                            1                                                                                            -                      1                                        /                    2                                                                                                              -                      1                                        /                    2                                                                                                0                                                                                            3                                        /                    2                                                                                                              -                                              3                                                              /                    2                                                                        ]                                              (        1        )                                          [                      C                          dq              ⁢              _              ⁢              inv                                ]                =                              [                          C              dq                        ]                                -            1                                              (        2        )            
The limiters LMT1 and LMT2 are provided at the output of the PI control for stabilizing the current control, and thereafter the neutral point potential compensation quantity V_cmp is added to the three-phase command voltage V_cmd in the configuration of FIG. 7. The corrected command voltage V_cmd′ after the addition is inputted to the limiter LMT3 before the PWM process. When the corrected command voltage V_cmd′ reaches a threshold value, the limiter processed command voltage V_cmd″ is subjected to the voltage limitation. Therefore, the effective value of the output voltage of the power conversion apparatus (the voltage between U and V terminals of FIG. 6, for example) is unable to retain the linearity. Moreover, when the time of the voltage limitation becomes long, the effective value of the output current of the power conversion apparatus too loses the linearity and the operation becomes unstable.
This is because the current control includes the integral action. The integral action functions to amplify the deviation with elapse of time. However, because of the limitation of the output voltage, the deviation is not reduced, and the integral action becomes excessive for amplifying the command output voltage. Consequently, the effective value of the output current loses the linearity. This phenomenon is referred to as windup. When the output currents Iu, Iv and Iw at the time of occurrence of windup are observed on the frequency axis, there appears an offset of a wide bandwidth of several Hz to several hundreds Hz. That is, many undesired fluctuation or vibrations are involved in the currents.
The current control is performed on the d, q axes. However, the system does not work properly by performing the anti-windup process (for suppressing the windup) of the ACR directly without modification. The corrected command voltage V_cmd′ is produced by superimposing the neutral point potential compensation quantity V_cmp on the three-phase command voltage V_cmd, and the command voltage limitation is performed by the limiter LMT3. Consequently, the corrected command voltage V_cmd′ reaches the threshold voltage, the voltage limitation is imposed on the limiter processed command voltage V_cmd″, and hence the system cannot perform the anti-windup process correctly.
In the illustrated example, the limiters LMT1 and LMT2 are provided on the output side of the current control section 2. In general, the capacitances of smoothing capacitors Cdc1 and Cdc2 shown in FIG. 6 are set equal to each other. However, the initial charge quantities of smoothing capacitors Cdc1 and Cdc2 are not equal to each other because of nonuniformity caused by the production process and aging degradation. Therefore, there arises a deviation between the dc voltages Vdc1 and Vdc2, and the neutral point potential compensation quantity V_cmp is increased. Especially, the motor M requires a large output current at the time of start of the motor. Therefore, the superimposition of the neutral point potential compensation quantity V_cmp at the time of start tends to cause an increase of the corrected command voltage V_cmd′ to the threshold value and the limitation on the limiter processed command voltage V_cmd″, and thereby impedes the stable operation. It is possible to avoid the limitation on the limiter processed command voltage V_cmd″ by decreasing the gain of the current control or the neutral point potential control. However, the trade-off for avoiding the limitation is a decrease of the control performance in the steady state. Accordingly, a control mechanism is required for considering interference between the current control and the neutral point potential control without decreasing the gain of the neutral point potential control.
Furthermore, in the case in which the load of the power conversion apparatus is an induction machine or a synchronous machine, the occurrence of the windup could cause fluctuation of torque like the fluctuation of the output current.
As explained above, the problem is to suppress interference between the current control and the neutral point potential control in the three-phase neutral point clamped power conversion apparatus performing the PWM control.